The server.json file is a file that contains variable data, such as the current allocation UUID, for each server. It’s automatically generated and populated for each server based on the configuration variables and the server data. It contains all the built-in configuration variables and any custom values from your build configuration variables.
The file itself can be reached by passing in /mnt/unity/server.json as the argument for a command-line variable.
You can read more in our documentation here